Lateral epicondylitis (LE), or tennis elbow, is a common and often disabling condition affecting young and middle-aged adults-most commonly manual workers and recreational athletes. It results in significant pain and disability, limiting work productivity and an individual's ability to participate in and enjoy recreational activities. Like many other chronic musculoskeletal conditions LE often shows an incomplete response to acute treatments, leading to chronic pain and disability. Prolotherapy (PrT) involves the injection of an irritant or proliferant solution into tendons, ligaments, and joints to treat chronic musculoskeletal pain. We will be conducting a 3-group randomized, blinded trial to determine the safety and efficacy of PrT injections in the treatment of chronic LE. Additionally, we will seek to determine to what extent any beneficial effect of PrT is derived from deep needle placement vs. a specific effect of the injectate. The specific aims for this study are: 1) to determine if PrT is a safe, well-tolerated, and effective treatment for individuals with chronic LE;and 2) to determine whether the deep needle placement alone is responsible for the therapeutic effect, independent of injectate. In order to accomplish these aims, we will measure indicators of pain and disability using self-rating scales, questionnaires and physical measures. Subjects will be stratified prior to randomization on two factors: 1) unilateral vs. bilateral LE;and 2) treatment with steroids within the prior 6 months. If effective, subjects receiving PrT will show greater reduction of pain and functional impairment compared with the other two groups. This research will guide future studies on prolotherapy by determining what may be the best control condition. Additionally, further studies may explore the mechanism responsible for any beneficial effect. Public Health Relevance: Lateral epicondylitis is a common and often disabling condition, most prominently affecting manual laborers and recreational athletes, with chronicity occurring in as many as 50% of individuals. Prolotherapy is a safe and easily administered injection technique that is commonly used to treat musculoskeletal pain, but studies have been limited and inconclusive. This randomized placebo controlled study will explore the possible efficacy of prolotherapy for lateral epicondylitis and determine an optimal control procedure for future prolotherapy research.